Embodiments relate to subterranean operations and, in certain embodiments, to settable compositions and methods of using settable compositions in subterranean formations.
During the drilling of a wellbore in a subterranean formation, a drilling fluid may be used to, among other things, cool the drill bit, lubricate the rotating drill string to prevent it from sticking to the walls of the well bore, prevent blowouts by serving as a hydrostatic head to counteract the sudden entrance into the well bore of high pressure formation fluids, and remove drill cuttings from the well bore. A drilling fluid may be circulated downwardly through a drill pipe and drill bit and then upwardly through the wellbore to the surface. The drilling fluid used may be any number of fluids (gaseous or liquid) and mixtures of fluids and solids (such as solid suspensions, mixtures, and emulsions).
After drilling to a desired depth and prior to and in preparation of the cementing phase, the drill bit may be withdrawn from the wellbore, and circulation of the drilling fluid is stopped. The drilling fluid may be left in the wellbore along with a filter cake of solids from the drilling fluid. Next, a pipe string (e.g., casing, liners, etc.) may be introduced into the well bore. Depending on the depth of the well bore and whether or not any problems are encountered in introducing the pipe string into the well bore, the drilling fluid may remain relatively static in the well bore for a relatively long time period, for example, up to about 2 weeks or longer. While drilling fluids are generally not settable (e.g., they generally do not to form a hardened mass over time), drilling fluids may increase in gel strength over time. Accordingly, during the time period that the drilling fluid is static in the well bore, portions of the drilling fluid may increase in gel strength so that displacement of the drilling fluid from within the well bore may be become more difficult. At a desired time, the pipe string may be cemented in place by pumping a cement composition through the pipe string and into the annulus between the pipe string and the walls of the well bore whereby the drilling fluid in the annulus is displaced therefrom by the cement composition. While a variety of techniques have been developed for improving the displacement of the drilling fluid from the annulus, if the drilling fluid has developed gel strength due to remaining static in the well bore for a long period of time, portions of the drilling fluid in the well bore are bypassed by the cement composition. Since the drilling fluid is not settable, i.e., it does not set into a rigid sealable mass, formation fluids enter and flow in the well bore which is highly undesirable.                In some instances, a settable composition (commonly referred to as a “settable spotting composition”) may be used to remove drilling fluid and prevent the drilling fluid filter cake from interfering with subsequent primary cementing operations. These settable spotting compositions may be used to at least partially displacing the drilling fluid before the drilling fluid in the wellbore has had a chance to gain significant gel strength, for example, prior to introducing the pipe string into the well bore. Generally, these settable spotting compositions should not have an undesirable increase in gel strength after being static in the wellbore for a period of time, for example, up to at least two weeks, so that the settable spotting compositions may be displaced from the wellbore. After the wellbore is at least partially filled with the settable spotting composition, the pipe string to be cemented may be introduced into the wellbore. When the cement composition is pumped through the pipe string into the annulus, the drilling fluid (if any) and settable spotting composition in the pipe string and annulus should be displaced ahead of the cement composition. The settable spotting composition, if any, remaining in fractures or other permeable portions of the subterranean formation should set into a hardened mass, thereby preventing or reducing the entry or flow of formation fluids in the annulus.        
In alternative operations, commonly referred to as “puddle jobs,” a settable composition may be placed into the wellbore before the casing and consequently the settable composition must remain in a fluid puddle state long enough for the casing string to be placed into the wellbore. Once the casing string is successfully positioned the settable composition may then set into a hardened mass, which may be sealable and/or may prevent the migration of fluids in the wellbore.
While settable compositions have been developed heretofore, challenges exist with their successful use in subterranean cementing operations. For example, settable compositions used as settable spotting compositions should ideally remain fluid long enough so that they can ultimately be displaced with the cement composition or any associated spacer fluids. Similarly, settable compositions used in puddle jobs should ideally remain fluid long enough to place and position the casing within the wellbore. Moreover, it may be desirable for the settable compositions to develop a sufficient compressive strength when it is no longer desired for the settable composition to remain fluid.